■ °\: ' 5S. 
 
 TECHNICAL NOTE number 180 
 
 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE 
 
 FOREST PRODUCTS LABORATORY 
 
 madison 5, Wisconsin revised December 1952 
 
 COMPARATIVE STRENGTH. OF AIR-DRIED 
 AND KILN-DRIED J WOOD 
 
 L_J^_££P0SIT0RY 
 
 Some wood users claim that kiln-dried wood is brash and not equal in 
 strength to wood that is air dried. Others advance figures purporting 
 to show that kiln-dried wood is much stronger than air -dried wood. But 
 some 150, 000 comparative strength tests, made by the Forest Products 
 Laboratory, of the U. S. Forest Service, on kiln-dried and air-dried 
 specimens of 28 common species of wood show that good kiln drying and 
 good air drying have the same effect upon the strength of the wood. 
 
 The belief that kiln drying produces stronger wood than air drying is usu- 
 ally the result of failure to consider differences in moisture content. The 
 moisture content of wood on leaving the kiln is generally from 2 to 6 per- 
 cent lower than that of thoroughly air-dried wood. Since wood rapidly 
 increases in strength with loss of moisture, higher strength values may 
 be obtained temporarily from kiln-dried wood that has not come into equi- 
 librium with the atmosphere than would be obtained from air -dried wood. 
 Such a difference in strength has no real significance, however, since 
 in use apiece of wood will come to practically the same moisture condi- 
 tion whether it is kiln dried or air dried. 
 
 It must be emphasised that the appearance of the dried wood is not a reli- 
 able criterion of the effect the drying process has had upon its strength. 
 The strength properties maybe seriously injured by the use of excessive 
 temperatures without visible damage to the wood. Also, it has been 
 found that the same kiln-drying process cannot be applied with equal sue - 
 cess to all species. To insure uninjured kiln-dried material, an effici- 
 ent kiln, a skilled operator with a knowledge of the correct kiln condi- 
 tions to use with a stock of a given species, grade, and thickness, and 
 a record showing that no more severe treatment has been employed, are 
 necessary. 
 
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